The constant are chosen so that D(i,j) = K(i,i) + K(j,j) - 2 K(i,j)
where D is the metric corresponding to the kenrel (if the arguments
are non-negative vectors). Each kernel can be interpreted as the
inner product inducing the corresponding metric in an embedding of
the real space into an approrpiate reproducing Kenrel Hilbert
space.

VL_ALLDIST2() supports several storage classes. X and Y must have the
same storage class. The sotrage class of D is promoted to reduce
the chance of overvlow, but this is not checked.